Methods for encapsulating nanocrystals and resulting compositions

ABSTRACT

The present invention provides methods for hermetically sealing luminescent nanocrystals, as well as compositions and containers comprising hermetically sealed luminescent nanocrystals. By hermetically sealing the luminescent nanocrystals, enhanced lifetime and luminescence can be achieved.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 14/858,585, filed Sep. 18, 2015, which is a divisional of U.S.patent application Ser. No. 14/194,996, filed Mar. 3, 2014, which is adivisional of U.S. patent application Ser. No. 13/684,782, filed Nov.26, 2012, which is a divisional of U.S. patent application Ser. No.12/318,516, filed Dec. 30, 2008, each of which are incorporated hereinby reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to methods for hermetically sealingluminescent nanocrystals, and hermetically sealed nanocrystalcompositions. The present invention also provides microspherescomprising luminescent nanocrystals as well as methods of making themicrospheres.

Background of the Invention

Luminescent nanocrystals when exposed to air and moisture undergooxidative damage, often resulting in a loss of luminescence. The use ofluminescent nanocrystals in areas such as down-conversion and filteringlayers, as well as other applications, often expose luminescentnanocrystals to elevated temperatures, high intensity light,environmental gasses and moisture. These factors, along withrequirements for long luminescent lifetime in these applications, oftenlimits the use of luminescent nanocrystals or requires frequentreplacement.

BRIEF SUMMARY OF THE INVENTION

There exists a need therefore for methods and compositions tohermetically seal luminescent nanocrystals, thereby allowing forincreased usage lifetime and luminescent intensity. The presentinvention fulfills these needs.

The present invention provides methods and compositions for hermeticallysealing luminescent nanocrystals. The compositions prepared according tothe present invention can be applied to a variety of applications, andthe methods allow for preparation of various shapes and configurationsof hermetically sealed nanocrystal compositions.

In one embodiment, the present invention provides methods ofhermetically sealing one or more compositions comprising a plurality ofluminescent nanocrystals. In exemplary embodiments, a first substrate isprovided, and one or more compositions comprising a plurality ofluminescent nanocrystals are disposed onto the first substrate (forexample, via screen printing). A second substrate is disposed on thefirst substrate so as to cover the compositions of luminescentnanocrystals. The first and second substrates are then sealed.

In exemplary embodiments, the first and second substrates are glasssubstrates, and suitably, the substrates have one or more recessesformed therein. In further embodiments, the first substrate furthercomprises a third substrate having one or more recesses formed therein.

Suitably, the luminescent nanocrystals for use in the practice of thepresent invention are core-shell luminescent nanocrystals, such asCdSe/ZnS, CdSe/CdS or InP/ZnS nanocrystals, and suitably are about 1-10nm in size.

Suitably, the first and second substrates are sealed with a polymericsealant, such as an epoxy sealant. In exemplary embodiments, theluminescent nanocrystal compositions are cured prior to sealing. Insuitable embodiments, the compositions are separated from each otherfollowing the sealing of the first and second substrates.

The methods of the present invention can further comprise disposing abarrier layer on the first and second substrates, such as an inorganiclayer, for example a layer of SiO₂, TiO₂ or AlO₂. The barrier layers aresuitably disposed by atomic layer deposition or sputtering.

In further embodiments, the methods of the present invention compriseforming one or more recesses in and/or on the first substrate. The oneor more compositions comprising a plurality of luminescent nanocrystalsare then disposed into the recesses, and the second substrate isdisposed on the first substrate so as to cover the compositions ofluminescent nanocrystals prior to sealing.

In exemplary embodiments, the first substrate is etched so as to formone or more recesses. In further embodiments, a third substrate havingone or more recesses formed therein is disposed onto the firstsubstrate. In additional embodiments, a third substrate is disposed ontothe first substrate and one or more recesses are etched into the thirdsubstrate. In still further embodiments, third substrate is disposedonto the first substrate so as to form one or more recesses on thesurface of the first substrate.

The present invention also provides hermetically sealed compositionsprepared by the various methods described throughout.

In further embodiments, the present invention provides microspheres.Suitably, the microspheres comprise a central region, a first layer onan outer surface of the central region, the first layer comprising oneor more luminescent nanocrystals, and a barrier layer on an outersurface of the first layer.

Suitably, the central region of the microspheres comprises silica, andthe first layer comprises an inorganic material, such as silica ortitania. Exemplary luminescent nanocrystals, including core-shellnanocrystals, are described throughout. Suitably, the barrier layercomprises an inorganic layer, such as SiO₂, TiO₂ or AlO₂.

In exemplary embodiments, the microspheres have a diameter of less thanabout 500 microns, suitably less than about 10 microns, more suitablyless than about 1 micron.

The present invention also provides method of forming microspheres.Suitably, a particle comprising a first inorganic material is provided,and the particle is contacted with a composition comprising a precursorto a second inorganic material and one or more luminescent nanocrystals.A peripheral region is formed on an outer surface of the particle, theperipheral region comprising the second inorganic material and theluminescent nanocrystals. Then, a barrier layer is disposed on an outersurface of the peripheral region.

Additional features and advantages of the invention will be set forth inthe description that follows, and in part will be apparent from thedescription, or may be learned by practice of the invention. Theadvantages of the invention will be realized and attained by thestructure and particularly pointed out in the written description andclaims hereof as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and areintended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIGS. 1A-1D show a method of hermetically sealing luminescentnanocrystals in accordance with an embodiment of the present invention.

FIG. 1E shows a flowchart of a method of hermetically sealingluminescent nanocrystals in accordance with an embodiment of the presentinvention.

FIGS. 2A-2G show a method of hermetically sealing luminescentnanocrystals in accordance with an embodiment of the present invention.

FIGS. 3A-3C show separating hermetically sealed luminescent nanocrystalsin accordance with an embodiment of the present invention.

FIG. 4 shows a flowchart of a method of hermetically sealing luminescentnanocrystals in accordance with an embodiment of the present invention.

FIG. 5 shows a microsphere in accordance with an embodiment of thepresent invention.

FIG. 6 shows a flowchart of a method of preparing a microsphere inaccordance with an embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. In the drawings, like reference numbers indicateidentical or functionally similar elements.

DETAILED DESCRIPTION OF THE INVENTION

It should be appreciated that the particular implementations shown anddescribed herein are examples of the invention and are not intended tootherwise limit the scope of the present invention in any way. Indeed,for the sake of brevity, conventional electronics, manufacturing,semiconductor devices, and nanocrystal, nanowire (NW), nanorod,nanotube, and nanoribbon technologies and other functional aspects ofthe systems (and components of the individual operating components ofthe systems) may not be described in detail herein.

The present invention provides various compositions comprisingnanocrystals, including luminescent nanocrystals. The various propertiesof the luminescent nanocrystals, including their absorption properties,emission properties and refractive index properties, can be tailored andadjusted for various applications. As used herein, the term“nanocrystal” refers to nanostructures that are substantiallymonocrystalline. A nanocrystal has at least one region or characteristicdimension with a dimension of less than about 500 nm, and down to on theorder of less than about 1 nm. As used herein, when referring to anynumerical value, “about” means a value of ±10% of the stated value (e.g.“about 100 nm” encompasses a range of sizes from 90 nm to 110 nm,inclusive). The terms “nanocrystal,” “nanodot,” “dot” and “quantum dot”are readily understood by the ordinarily skilled artisan to representlike structures and are used herein interchangeably. The presentinvention also encompasses the use of polycrystalline or amorphousnanocrystals. As used herein, the term “nanocrystal” also encompasses“luminescent nanocrystals.” As used herein, the term “luminescentnanocrystals” means nanocrystals that emit light when excited by anexternal energy source (suitably light). As used herein when describingthe hermetic sealing of nanocrystals, it should be understood that insuitable embodiments, the nanocrystals are luminescent nanocrystals.

Typically, the region of characteristic dimension will be along thesmallest axis of the structure. Nanocrystals can be substantiallyhomogenous in material properties, or in certain embodiments, can beheterogeneous. The optical properties of nanocrystals can be determinedby their particle size, chemical or surface composition. The ability totailor the luminescent nanocrystal size in the range between about 1 nmand about 15 nm enables photoemission coverage in the entire opticalspectrum to offer great versatility in color rendering. Particleencapsulation offers robustness against chemical and UV deterioratingagents.

Nanocrystals, including luminescent nanocrystals, for use in the presentinvention can be produced using any method known to those skilled in theart. Suitable methods and exemplary nanocrystals are disclosed inPublished U.S. Patent Application No. 2008/0237540; U.S. Pat. No.7,374,807; U.S. patent application Ser. No. 10/796,832, filed Mar. 10,2004; U.S. Pat. No. 6,949,206; and U.S. Provisional Patent ApplicationNo. 60/578,236, filed Jun. 8, 2004, the disclosures of each of which areincorporated by reference herein in their entireties. The nanocrystalsfor use in the present invention can be produced from any suitablematerial, including an inorganic material, and more suitably aninorganic conductive or semiconductive material. Suitable semiconductormaterials include those disclosed in U.S. patent application Ser. No.10/796,832, and include any type of semiconductor, including groupII-VI, group III-V, group IV-VI and group IV semiconductors. Suitablesemiconductor materials include, but are not limited to, Si, Ge, Sn, Se,Te, B, C (including diamond), P, BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN,GaP, GaAs, GaSb, InN, InP, InAs, InSb, AlN, AlP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS,BeSe, BeTe, MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe,PbTe, CuF, CuCl, CuBr, CuI, Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂(S, Se,Te)₃, Al₂CO, and an appropriate combination of two or more suchsemiconductors.

In certain aspects, the semiconductor nanocrystals may comprise a dopantfrom the group consisting of: a p-type dopant or an n-type dopant. Thenanocrystals useful in the present invention can also comprise II-VI orIII-V semiconductors. Examples of II-VI or III-V semiconductornanocrystals include any combination of an element from Group II, suchas Zn, Cd and Hg, with any element from Group VI, such as S, Se, Te, Po,of the Periodic Table; and any combination of an element from Group III,such as B, Al, Ga, In, and Tl, with any element from Group V, such as N,P, As, Sb and Bi, of the Periodic Table.

The nanocrystals, including luminescent nanocrystals, useful in thepresent invention can also further comprise ligands conjugated,cooperated, associated or attached to their surface as describedthroughout. Suitable ligands include any group known to those skilled inthe art, including those disclosed in U.S. Pat. No. 7,374,807, U.S. Pat.No. 6,949,206 and U.S. Provisional Patent Application No. 60/578,236,the disclosures of each of which are incorporated herein by reference.Use of such ligands can enhance the ability of the nanocrystals toincorporate into various solvents and matrixes, including polymers.Increasing the miscibility (i.e., the ability to be mixed withoutseparation) of the nanocrystals in various solvents and matrixes allowsthem to be distributed throughout a polymeric composition such that thenanocrystals do not aggregate together and therefore do not scatterlight. Such ligands are described as “miscibility-enhancing” ligandsherein.

As used herein, the term nanocomposite refers to matrix materialscomprising nanocrystals distributed or embedded therein. Suitable matrixmaterials can be any material known to the ordinarily skilled artisan,including polymeric materials, organic and inorganic oxides.Nanocomposites of the present invention can be layers, encapsulants,coatings or films as described herein. It should be understood that inembodiments of the present invention where reference is made to a layer,polymeric layer, matrix, or nanocomposite, these terms are usedinterchangeably, and the embodiment so described is not limited to anyone type of nanocomposite, but encompasses any matrix material or layerdescribed herein or known in the art.

Down-converting nanocomposites (for example, as disclosed in U.S. Pat.No. 7,374,807) utilize the emission properties of luminescentnanocrystals that are tailored to absorb light of a particularwavelength and then emit at a second wavelength, thereby providingenhanced performance and efficiency of active sources (e.g., LEDs). Asdiscussed above, use of luminescent nanocrystals in such down-conversionapplications, as well as other filtering or coating applications, oftenexposes the nanocrystals to elevated temperatures, high intensity light(e.g., an LED source), external gasses, and moisture. Exposure to theseconditions can reduce the efficiency of the nanocrystals, therebyreducing useful product lifetime. In order to overcome this problem, thepresent invention provides methods for hermetically sealing luminescentnanocrystals.

Luminescent Nanocrystal Phosphors

While any method known to the ordinarily skilled artisan can be used tocreate nanocrystal phosphors, suitably, a solution-phase colloidalmethod for controlled growth of inorganic nanomaterial phosphors isused. See Alivisatos, A. P., “Semiconductor clusters, nanocrystals, andquantum dots,” Science 271:933 (1996); X. Peng, M. Schlamp, A.Kadavanich, A. P. Alivisatos, “Epitaxial growth of highly luminescentCdSe/CdS Core/Shell nanocrystals with photostability and electronicaccessibility,” J. Am. Chem. Soc. 30:7019-7029 (1997); and C. B. Murray,D. J. Norris, M. G. Bawendi, “Synthesis and characterization of nearlymonodisperse CdE (E=sulfur, selenium, tellurium) semiconductornanocrystallites,” J. Am. Chem. Soc. 115:8706 (1993), the disclosures ofwhich are incorporated by reference herein in their entireties. Thismanufacturing process technology leverages low cost processabilitywithout the need for clean rooms and expensive manufacturing equipment.In these methods, metal precursors that undergo pyrolysis at hightemperature are rapidly injected into a hot solution of organicsurfactant molecules. These precursors break apart at elevatedtemperatures and react to nucleate nanocrystals. After this initialnucleation phase, a growth phase begins by the addition of monomers tothe growing crystal. The result is freestanding crystallinenanoparticles in solution that have an organic surfactant moleculecoating their surface.

Utilizing this approach, synthesis occurs as an initial nucleation eventthat takes place over seconds, followed by crystal growth at elevatedtemperature for several minutes. Parameters such as the temperature,types of surfactants present, precursor materials, and ratios ofsurfactants to monomers can be modified so as to change the nature andprogress of the reaction. The temperature controls the structural phaseof the nucleation event, rate of decomposition of precursors, and rateof growth. The organic surfactant molecules mediate both solubility andcontrol of the nanocrystal shape. The ratio of surfactants to monomer,surfactants to each other, monomers to each other, and the individualconcentrations of monomers strongly influence the kinetics of growth.

In suitable embodiments, CdSe is used as the nanocrystal material, inone example, for visible light down-conversion, due to the relativematurity of the synthesis of this material. Due to the use of a genericsurface chemistry, it is also possible to substitutenon-cadmium-containing nanocrystals.

Core/Shell Luminescent Nanocrystals

In semiconductor nanocrystals, photo-induced emission arises from theband edge states of the nanocrystal. The band-edge emission fromluminescent nanocrystals competes with radiative and non-radiative decaychannels originating from surface electronic states. X. Peng, et al., J.Am. Chem. Soc. 30:7019-7029 (1997). As a result, the presence of surfacedefects such as dangling bonds provide non-radiative recombinationcenters and contribute to lowered emission efficiency. An efficient andpermanent method to passivate and remove the surface trap states is toepitaxially grow an inorganic shell material on the surface of thenanocrystal. X. Peng, et al., J. Am. Chem. Soc. 30:7019-7029 (1997). Theshell material can be chosen such that the electronic levels are type Iwith respect to the core material (e.g., with a larger bandgap toprovide a potential step localizing the electron and hole to the core).As a result, the probability of non-radiative recombination can bereduced.

Core-shell structures are obtained by adding organometallic precursorscontaining the shell materials to a reaction mixture containing the corenanocrystal. In this case, rather than a nucleation-event followed bygrowth, the cores act as the nuclei, and the shells grow from theirsurface. The temperature of the reaction is kept low to favor theaddition of shell material monomers to the core surface, whilepreventing independent nucleation of nanocrystals of the shellmaterials. Surfactants in the reaction mixture are present to direct thecontrolled growth of shell material and ensure solubility. A uniform andepitaxially grown shell is obtained when there is a low lattice mismatchbetween the two materials. Additionally, the spherical shape acts tominimize interfacial strain energy from the large radius of curvature,thereby preventing the formation of dislocations that could degrade theoptical properties of the nanocrystal system.

Exemplary materials for preparing core-shell luminescent nanocrystalsinclude, but are not limited to, Si, Ge, Sn, Se, Te, B, C (includingdiamond), P, Co, Au, BN, BP, BAs, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs,GaSb, InN, InP, InAs, InSb, AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb,ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, BeS, BeSe, BeTe,MgS, MgSe, GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbO, PbS, PbSe, PbTe, CuF,CuCl, CuBr, CuI, Si₃N₄, Ge₃N₄, Al₂O₃, (Al, Ga, In)₂ (S, Se, Te)₃, Al₂CO,and an appropriate combination of two or more such materials. Exemplarycore-shell luminescent nanocrystals for use in the practice of thepresent invention include, but are not limited to, (represented asCore/Shell), CdSe/ZnS, InP/ZnS, PbSe/PbS, CdSe/CdS, CdTe/CdS, CdTe/ZnS,as well as others.

Hermetically Sealed Luminescent Nanocrystal Compositions

In one embodiment, the present invention provides methods ofhermetically sealing one or more compositions comprising a plurality ofluminescent nanocrystals. As shown in flowchart 120 of FIG. 1E, withreference to the schematics in FIGS. 1A-1D, suitably the methodscomprise providing a first substrate 102 in step 122. In step 124, oneor more compositions 104 comprising a plurality of luminescentnanocrystals 106 are disposed onto the first substrate 102. In step 126,a second substrate 108 is disposed on the first substrate so as to coverthe compositions 104 of luminescent nanocrystals 106 as in FIG. 1B. Instep 128, the first and second substrates are then sealed.

As discussed throughout, the terms “hermetic,” “hermetic sealing,” and“hermetically sealed” are used to indicate that the compositions ofluminescent nanocrystals are prepared in such a way that the quantity ofgases (e.g., air) or moisture that passes through or penetrates thecontainer or composition, and/or that contacts the luminescentnanocrystals is reduced to a level where it does not substantiallyeffect the performance of the nanocrystals (e.g., their luminescence).Therefore, a “hermetically sealed composition,” for example one thatcomprises luminescent nanocrystals, is a composition that does not allowan amount of air (or other gas, liquid or moisture) to penetrate thecomposition and contact the luminescent nanocrystals such that theperformance of the nanocrystals (e.g., the luminescence) issubstantially effected or impacted (e.g., reduced).

As used throughout, a plurality of luminescent nanocrystals means morethan one nanocrystal (i.e., 2, 3, 4, 5, 10, 100, 1,000, 1,000,000, etc.,nanocrystals). The compositions will suitably comprise luminescentnanocrystals having the same composition, though in further embodiments,the plurality of luminescent nanocrystals can be various differentcompositions. For example, the luminescent nanocrystals can all emit atthe same wavelength, or in further embodiments, the compositions cancomprise luminescent nanocrystals that emit at different wavelengths.

Suitable matrixes for use in the compositions of the present inventioninclude polymers and organic or inorganic oxides. Suitable polymers foruse in the matrixes of the present invention include any polymer knownto the ordinarily skilled artisan that can be used for such a purpose.In suitable embodiments, the polymer is substantially translucent,transparent, or substantially transparent. Such polymers include, butare not limited to, poly(vinyl butyral):poly(vinyl acetate); epoxies;urethanes; silicone and derivatives of silicone, including, but notlimited to, polyphenylmethylsiloxane, polyphenylalkylsiloxane,polydiphenylsiloxane, polydialkylsiloxane, fluorinated silicones andvinyl and hydride substituted silicones; acrylic polymers and copolymersformed from monomers including but not limited to, methylmethacrylate,butylmethacrylate and laurylmethacrylate; styrene based polymers; andpolymers that are crosslinked with difunctional monomers, such asdivinylbenzene.

The luminescent nanocrystals used the present invention can be embeddedin a polymeric (or other suitable material, e.g., waxes, oils) matrixusing any suitable method, for example, mixing the nanocrystals in apolymer and casting a film, mixing the nanocrystals with monomers andpolymerizing them together, mixing the nanocrystals in a sol-gel to forman oxide, or any other method known to those skilled in the art. As usedherein, the term “embedded” is used to indicate that the luminescentnanocrystals are enclosed or encased within the polymer that makes upthe majority component of the matrix. It should be noted thatluminescent nanocrystals are suitably uniformly distributed throughoutthe matrix, though in further embodiments they can be distributedaccording to an application-specific uniformity distribution function.

In exemplary embodiments, first substrate 102 and second substrate 108are transparent, substantially transparent, or translucent substrate,such a polymer or a glass (e.g., a silica-comprising glass). Inexemplary embodiments, both first and second substrate comprise glass,though in other embodiments, one of the substrates can be glass and theother a polymeric material, or both can be polymeric materials. As shownin FIG. 1A, suitably first substrate 102 is of a size such that morethan one composition 104 of luminescent nanocrystals 106 can be disposedthereon. However, in additional embodiments, a single composition 104comprising a plurality of luminescent nanocrystals 106 be disposed on afirst substrate, and if desired, a plurality of first substrates canthen be used to prepare multiple hermetically sealed compositions. Thethickness of first substrate 102 is suitably on the order of about 1 μmto about 1 cm, suitably about 100 μm to about 100 mm. First and secondsubstrates are suitably the same size, though in other embodiments, theycan be different sizes, so long as the compositions are sealed by thesubstrates. Suitably, first and second substrates are on the order ofmillimeters to meters in at least one lateral dimension (i.e., in theplane of the substrate). Providing a first substrate 102 that istransparent, translucent or semi-transparent, allows light to passthrough substrate and contact the luminescent nanocrystals disposedthereon.

The thickness and size (e.g., area of coverage) of the compositions 104of the present invention that are disposed on the first substrate 102can be controlled by any method known in the art, such as spin-coating,screen printing, dip-coating, painting, spraying, etc. The luminescentnanocrystal compositions of the present invention can be any desirablesize, shape, configuration and thickness. For example, the compositionscan be disposed on the first substrate in the form of layers, as well asother shapes, for example, discs, drops, spheres, cubes or blocks,tubular configurations and the like. While the various compositions ofthe present invention can be any required or desired thickness,suitably, the compositions are on the order of about 1 μm to about 500μm in thickness (i.e., in one dimension). Suitably, the compositionshave at least one lateral dimension (i.e., in the plane of thesubstrate) that is in the range of about a few microns to centimeters.The luminescent nanocrystals can be embedded or dispersed in the variouscompositions/matrixes at any loading ratio that is appropriate for thedesired function. Suitably, the luminescent nanocrystals are loaded at aratio of between about 0.001% and about 75% by volume depending upon theapplication, matrix and type of nanocrystals used. The appropriateloading ratios can readily be determined by the ordinarily skilledartisan and are described herein further with regard to specificapplications. In exemplary embodiments, the amount of nanocrystalsloaded in a luminescent nanocrystal compositions are on the order ofabout 10% by volume, to parts-per-million (ppm) levels.

Luminescent nanocrystals for use in the present invention will suitablybe less than about 100 nm in size, and down to less than about 2 nm insize. In suitable embodiments, the luminescent nanocrystals of thepresent invention absorb visible light. As used herein, visible light iselectromagnetic radiation with wavelengths between about 380 and about780 nanometers that is visible to the human eye. Visible light can beseparated into the various colors of the spectrum, such as red, orange,yellow, green, blue, indigo and violet. The photon-filteringnanocomposites of the present invention can be constructed so as toabsorb light that makes up any one or more of these colors. For example,the nanocomposites of the present invention can be constructed so as toabsorb blue light, red light, or green light, combinations of suchcolors, or any colors in between. As used herein, blue light compriseslight between about 435 nm and about 500 nm, green light comprises lightbetween about 520 nm and 565 nm and red light comprises light betweenabout 625 nm and about 740 nm in wavelength. The ordinarily skilledartisan will be able to construct nanocomposites that can filter anycombination of these wavelengths, or wavelengths between these colors,and such nanocomposites are embodied by the present invention.

In other embodiments, the luminescent nanocrystals have a size and acomposition such that they absorb photons that are in the ultraviolet,near-infrared, and/or infrared spectra. As used herein, the ultravioletspectrum comprises light between about 100 nm to about 400 nm, thenear-infrared spectrum comprises light between about 750 nm to about 100μm in wavelength and the infrared spectrum comprises light between about750 nm to about 300 μm in wavelength.

While luminescent nanocrystals of any suitable material can be used inthe practice of the present invention, in certain embodiments, thenanocrystals are ZnS, InAs or CdSe nanocrystals, or the nanocrystalscomprise various combinations to form a population of nanocrystals foruse in the practice of the present invention. As discussed above, infurther embodiments, the luminescent nanocrystals are core/shellnanocrystals, such as CdSe/ZnS, CdSe/CdS or InP/ZnS.

As discussed throughout, the compositions 104 of luminescentnanocrystals 106 suitably comprise a polymeric substrate or matrix.Thus, the present invention comprises methods of hermetically sealingcompositions comprising luminescent nanocrystals, suitably polymericsubstrates comprising luminescent nanocrystals, by sealing thecompositions between a first and second substrates.

The ability to use polymeric substrates in the compositions 104 allowsfor the formation of various shapes and configurations of thecompositions, simply by molding, spreading, dropping, dispensing,spraying, layering, or otherwise manipulating the compositions into thedesired shape/orientation. For example, a solution/suspension ofluminescent nanocrystals can be prepared (e.g., luminescent nanocrystalsin a polymeric matrix). This solution can then be placed into anydesired mold to form a required shape, or can simply be disposed in ashape, and then cured (e.g., cooled or heated depending upon the type ofpolymer) to form a solid or semi-solid structure. For example, as shownin FIG. 1A, the compositions can be disposed in the shapes of disks ordroplets.

In exemplary embodiments, the compositions 104 comprising luminescentnanocrystals 106 (note, figures are not to scale) are disposed onsubstrate 102 in a high-throughput format, for example, by using screenprinting, ink-jet printing, or other application technique that deposita large number of individual samples onto a substrate.

In suitable embodiments, the sealing in step 128 of flowchart 120comprises sealing with a polymeric sealant. Suitable polymeric sealantsthat can be used in the practice of the present invention are well knownin the art, and are those which when dried or cured, are transparent, orat least semitransparent, or translucent. Exemplary polymeric sealantswhich can be utilized include, but are not limited to, silicones,epoxies, various rubbers, various acrylics, etc. In addition to suitablybeing transparent or at least translucent, the sealant should also beimpermeable, or at least substantially impermeable, to air and moisture,so as to hermetically seal the first and second substrates.

Suitably, the first 102 and second 108 substrates are sealed byintroducing sealant 110 to the first and second substrates, for example,by pouring, dipping, wicking, painting, injecting, etc., sealant 110,such that the sealant forms a seal 112 between the first and secondsubstrates. Suitably, the luminescent nanocrystal composition is cured(e.g., via heating or cooling) prior to the sealing with the sealant.

In further embodiments, as shown in FIGS. 2A-2B, first substrate 102suitably comprises one or more recesses 202 formed, at least one of, inand on, the substrate. As used herein, a “recess” refers to a hole,indentation, well, crack, imperfection, or other depression in and/or onsubstrate 102. Forming the recesses, at least one of, in and on, meansthat the recesses are formed in and/or on, the substrate 102. A recess“on” first substrate 102 refers to a recess that is above the surface offirst substrate 102, for example, a recess formed in a third substrateas described herein. A recess that is “in” first substrate 102 refers toa recess that penetrates into the surface of first substrate 102 to anydepth. Note that recesses can be formed both in and on the substrate inthe same composition, or can be formed only in, or only on, thesubstrate 102.

Suitably, recesses in substrate 102 will not pass through the entiresubstrate, but instead have a depth into the substrate that is less thanthe entire thickness of the substrate, thereby providing a reservoir forreceipt of compositions 104. Suitably, the recesses 202 are on the orderof about 0.5 mm to about 10 mm in at least one lateral dimension (adimension in the plane of first substrate 102, e.g., diameter if acircular-shaped recess is utilized), more suitably about 1 mm to about10 mm, about 1 mm to about 9 mm, about 1 mm to about 8 mm, about 1 mm toabout 7 mm, about 1 mm to about 6 mm, about 1 mm to about 5 mm, about 1mm to about 4 mm, about 1 mm to about 3 mm, about 1 mm to about 2 mm, orabout 10 mm, about 9 mm, about 8 mm, about 7 mm, about 6 mm, about 5 mm,about 4 mm, about 3 mm, about 2 mm, or about 1 mm, in at least onelateral dimension.

Recesses will suitably be separated by sections of substrate 102 (orother materials as described herein) so that they are on the order ofabout 0.1 mm to about 10 mm apart (edge-to-edge separation). Suitably,recesses 202 are separated by distances of about 1 mm to about 10 mm,about 1 mm to about 9 mm, about 1 mm to about 8 mm, about 1 mm to about7 mm, about 1 mm to about 6 mm, about 1 mm to about 5 mm, about 1 mm toabout 4 mm, about 1 mm to about 3 mm, about 1 mm to about 2 mm, or about10 mm, about 9 mm, about 8 mm, about 7 mm, about 6 mm, about 5 mm, about4 mm, about 3 mm, about 2 mm, or about 1 mm.

The depth of recesses 202 into the surface of substrate 102 (i.e., thedistance into the substrate normal to the surface of the substrate) ispartially dictated by the thickness of substrate 102, though the depthsuitably extends only a portion of the way into the surface of substrate102. In exemplary embodiments, the depth of recesses 202 is on the orderof about 100 μm to about 100 mm, suitably about 500 μm to about 10 mm.While in exemplary embodiments the depth of recesses 202 can be uniformacross the recess, in other embodiments, the recess can have a slopingor non-inform depth.

While in exemplary embodiments, recesses 202 have a circularcross-section, in other embodiments, any shape can be used, e.g.,rectangular, square, triangular, irregular, etc.

As shown in a further embodiment in FIG. 2F, first substrate 102 canfurther comprise a third substrate 204 that has one or more recesses 202formed into the third substrate 204. Suitably, the recesses in the thirdsubstrate will pass all the way through to the surface of firstsubstrate 102 (in suitable embodiments, surface 102 may also haverecesses therein), though in other embodiments, recesses 202 in thirdsubstrate 204 will not pass all the way through the third surface. Thus,as shown in FIG. 2F, recesses 204 can be in the form of cylinders (orother suitable shapes, e.g., rectangles, squares, irregular shapes,etc.). The thickness of third substrate is suitably on the order ofabout 100 μm to about 100 mm, suitably about 500 μm to about 10 mm, orabout 500 μm to about 5 mm.

In exemplary embodiments, third substrate 204 comprises a polymericmaterial, including a photoresistant materials. The use of aphotoresistant material allows for masking and etching to producerecesses 202 in the third substrate 204 (as described herein). Examplesof methods of the use of photoresistant materials, as well asphotoresist developers, can be found in, for example, Sze, S. M.,“Semiconductor Devices, Physics and Technology,” John Wiley & Sons, NewYork, pp. 436-442 (1985), the disclosure of which is incorporated byreference herein in its entirety. In general, photoresists (such asnegative photoresists) for use in the practice of the present inventioncomprise a polymer combined with a photosensitive compound. Uponexposure to radiation (e.g., UV light), the photosensitive compoundcross-links the polymer, rendering it resistant to a developing solvent.Unexposed areas, however, are removable by the developing solvent. Someexemplary negative photoresist materials and developers include Kodak®747, copolymer-ethyl acrylate and glycidylmethacrylate (COP), GeSe andpoly(glycidyl methacrylate-co-ethyl acrylate) DCOPA. Disposing ofnegative photoresist material can be performed using any suitablemethod, for example, spin coating, spray coating, or otherwise layeringthe material. In contrast, “positive photoresistant” materials becomeless chemically robust when exposed to radiation, and hence, work in theopposite manner to negative photoresistant materials. Here, materialsthat are exposed to radiation will remain to generate the mask, whileunexposed areas will be removed.

As shown in FIGS. 2B and 2C, compositions 104 comprising luminescentnanocrystals are disposed in the recesses 202. Suitably, the recessesare filled such that there is no, or very little, gap between the top ofthe composition 104 and the surface of the substrate 102. This providesfor a tight seal between the second substrate 108 and the firstsubstrate 102, as shown in FIG. 2C-2E, when sealed with sealant 110,thereby providing hermetically sealed luminescent nanocrystals. When athird substrate 204 comprising recesses 202 is utilized, suitably thecompositions 104 are disposed in the recesses so that there is no, orvery little, gap between the top of the composition and the surface ofthe third substrate 204.

In further embodiments, as shown FIG. 1E, the methods of the presentinvention can further comprise step 130, in which a barrier layer (notshown) is disposed on the surface of the first 102 and second substrates108. As used herein, the term “barrier layer” is used to indicate alayer, coating, sealant or other material that is disposed on the firstand second substrates. Such barrier layers provide an additional measureof hermetic sealing above and beyond the hermetic sealing provided bysealing of the first and second substrates.

Examples of barrier layers include any material layer, coating orsubstance that can create an airtight seal on thesubstrates/compositions. Suitable barrier layers include inorganiclayers, suitably an inorganic oxide such as an oxide of Al, Ba, Ca, Mg,Ni, Si, Ti or Zr. Exemplary inorganic oxide layers, include SiO₂, TiO₂,AlO₂ and the like. As used throughout, the terms “dispose,” and“disposing” include any suitable method of application of a barrierlayer. For example, disposing includes layering, coating, spraying,sputtering, plasma enhanced chemical vapor deposition, atomic layerdeposition, or other suitable method of applying a barrier layer to thesubstrates/compositions. In suitable embodiments, sputtering is used todispose the barrier layer on the substrates/compositions. Sputteringcomprises a physical vapor deposition process where high-energy ions areused to bombard elemental sources of material, which eject vapors ofatoms that are then deposited in thin layers on a substrate. See forexample, U.S. Pat. Nos. 6,541,790; 6,107,105; and 5,667,650, thedisclosures of each of which are incorporated by reference herein intheir entireties.

In further embodiments, disposing the barrier layer can be carried outusing atomic layer deposition. In order to properly hermetically sealthe nanocrystal composition, a virtually defect-free (i.e., pinhole-free) barrier layer is often required. In addition, application ofthe barrier layer should not degrade the polymer, substrates and/or thenanocrystals. Therefore, in suitable embodiments, atomic layerdeposition is used to dispose the barrier layer.

Atomic layer deposition (ALD) can comprise disposition of an oxide layer(e.g., TiO₂, SiO₂, AlO₂, etc.) on the substrates/compositions, or infurther embodiments, deposition of a non-conductive layer, such as anitride (e.g., silicon nitride) can be used. ALD deposits an atomiclayer (i.e., only a few molecules thick) by alternately supplying areaction gas and a purging gas. A thin coating having a high aspectratio, uniformity in a depression, and good electrical and physicalproperties, can be formed. Barrier layers deposited by the ALD methodsuitably have a low impurity density and a thickness of less than 1000nm, suitably less than about 500 nm, less than about 200 nm, less thanabout 50 nm, less than about 20 nm, or less than about 5 nm.

For example, in suitable embodiments, two reaction gases, A and B areused. When only the reaction gas, A, flows into a reaction chamber,atoms of the reaction gas A are chemically adsorbedsubstrates/compositions. Then, any remaining reaction gas A is purgedwith an inert gas such as Ar or nitrogen. Then, reaction gas B flows in,wherein a chemical reaction between the reaction gases A and B occursonly on the surface of the substrates/compositions on which the reactiongas A has been adsorbed, resulting in an atomic barrier layer on thesubstrates/compositions.

In embodiments where a non-conductive layer, such as a nitride layer isdisposed, suitably SiH₂Cl₂ and remote plasma enhanced NH₃ are used todispose a silicon nitride layer. This can be performed at a lowtemperature and does not require the use of reactive oxygen species.

Use of ALD for disposition of a barrier layer on thesubstrates/compositions generates a virtually pin-hole free barrierlayer regardless of the morphology of the substrate. The thickness ofthe barrier layer can be increased by repeating the deposition steps,thereby increasing the thickness of the layer in atomic layer unitsaccording to the number of repetitions. In addition, the barrier layercan be further coated with additional layers (e.g., via sputtering, CVDor ALD) to protect or further enhance the barrier layer.

Suitably, the ALD methods utilized in the practice of the presentinvention are performed at a temperature of below about 500° C.,suitably below about 400° C., below about 300° C., or below about 200°C.

Exemplary barrier materials include organic material designed tospecifically reduce oxygen and moisture transmission. Examples includefilled epoxies (such as alumina filled epoxies) as well as liquidcrystalline polymers.

As shown in flowchart 120 of FIG. 1E, the methods of the presentinvention suitably further comprise separating the one or morehermetically sealed compositions from each other following sealing ofthe substrate layers, as shown in FIGS. 3A-3C. This separation can bebefore or after the disposing of a barrier layer, though suitably thebarrier layer, if utilized, is disposed after the separation.

As shown in FIGS. 3A-3C, a hermetically sealed structure 302 comprisingmultiple, individually sealed compositions can be separated intosub-structures 304, or suitably further into individual structures 306,each comprising a single hermetically sealed composition, which initself comprises a plurality of luminescent nanocrystals. Thus,preparation of a plurality of sealed compositions can lead toindividual, separated compositions.

Methods for separating the hermetically sealed compositions from eachother include various methods well known in the art, such as viamechanical dicing (e.g., via knife, wedge, saw, blade, or other cuttingdevice), via a laser, via water jet, etc.

In further embodiments, the present invention provides additionalmethods of hermetically sealing one or more compositions of luminescentnanocrystals. As shown in flowchart 400 of FIG. 4, with reference toFIGS. 2A-2G, in exemplary embodiments, the methods comprise step 402, inwhich a first substrate 102 is provided. In step 404 of flowchart 400,one or more recesses 202 are generated in and/or on the first substrate.

In step 406 of flowchart 400, one or more compositions 104 comprising aplurality of luminescent nanocrystals 106 are disposed into the recesses204. In step 408, a second substrate 108 is then disposed on the firstsubstrate 102 so as to cover the compositions 104 of luminescentnanocrystals 106. In step 410 of flowchart 400, the first and secondsubstrates are then sealed 112.

As described throughout, suitably substrates 102 and 108 aretransparent, semi-transparent or translucent substrates, such as polymeror glass substrates. The size and thickness of substrates 102 and 108are described throughout.

Step 404 of flowchart 400 comprises generating one or more recesses 202in and/or on the first substrate 102. In exemplary embodiments, recesses202 are generated directly in the surface of first substrate 102. Thatis, material is removed from the surface of first substrate 102 so as togenerate recesses 202. Methods for removing material from firstsubstrate 102 include etching (e.g., chemical etching using variousacids or other etchants, including those disclosed herein), gouging,cutting, whittling, drilling, etc.

In further embodiments, recesses 202 can be generated on first substrate102. In such embodiments, a third substrate 204 is suitably disposed onfirst substrate 102. Recesses 202 are then generated in the thirdsubstrate, for example, by etching (e.g., chemical etching using variousacids), gouging, cutting, whittling, drilling, etc., into the substrate.Suitably, a masking/etching method is used to generate recesses in thethird substrate. In further embodiments, recesses 202 can be generatedby disposing a previously prepared third substrate in which recess havealready been generated. In still further embodiments, recesses can beformed on the surface of first substrate 102 by disposing and arrangingthird substrate sections 206 on first substrate 102, wherein recesses202 are generated or formed within the gaps/spaces between the sections,as shown in FIG. 2G.

Exemplary compositions comprising luminescent nanocrystals (e.g.,polymeric compositions/matrixes) as well as suitable nanocrystals aredescribed throughout. Suitably, the luminescent nanocrystals arecore-shell luminescent nanocrystals, such as CdSe/ZnS, CdSe/CdS andInP/ZnS. Exemplary sizes of nanocrystals are described herein, andsuitably, the luminescent nanocrystals are between about 1-10 nm insize. Methods for disposing the compositions of luminescent nanocrystalsin the recesses are described throughout, and include screen printingand other methods to generate a high-throughput deposition.

As described throughout, suitably second substrate is a transparent,semi-transparent or translucent substrate, such as a polymeric materialor a glass. Hermetically sealing the compositions of luminescentnanocrystals between two glass substrates allows the nanocrystals to beutilized in various applications, such as in down-conversion in LEDs, asdescribed herein.

As described throughout, suitably the first and second substrates aresealed with a polymeric sealant, such as a silicon-based, epoxy-based oracrylic-based sealant. The sealant can be introduced 110 to the firstand second substrates using any suitable method, such as pouring thesealant over the substrates (and then squeezing out residual by applyingpressure to the substrates), wicking the substrate into space betweenthe substrates, injecting the sealant, dipping the substrates in asealant, and other suitable methods. In other embodiments, a sealant cansimply be disposed on the outside edges of the first and secondsubstrates, for example, by painting, spraying, spreading or otherwiseapplying the sealant without requiring the sealant to penetrate betweenthe first and second substrates.

As shown in FIG. 4, suitably, the luminescent nanocrystals are cured instep 412 prior to sealing the first and second substrates in step 410,though in additional embodiments, the substrates can be sealed and thenthe compositions of luminescent nanocrystals can be cured.

The methods of the present invention can also further comprise step 414of flowchart 400, of disposing a barrier layer on the first and secondsubstrates to further hermetically seal the substrates. Methods ofdisposing a barrier layer (e.g., atomic layer deposition, sputtering,etc.) are described throughout, as are exemplary barrier layers,including inorganic layers, such as layers comprising SiO₂, TiO₂ orAlO₂.

As shown in flowchart 400, the methods suitably further comprise step416, in which the hermetically sealed compositions are separated fromeach other, as shown in FIGS. 3A-3C, for example. The separation canoccur before of after the barrier layer is disposed. As describedherein, the methods provided allow for a high-throughput generationindividual, separate samples of luminescent nanocrystals that can beused in various applications, such as in LEDs, displays, etc.

The present invention also provides hermetically sealed compositionsprepared by the various methods described herein. Exemplarycompositions, sizes and characteristics of the luminescent nanocrystals,as well as the substrates, sealants and other components (e.g., barrierlayers) of the sealed compositions are described throughout.

In suitable embodiments of the present invention, the various steps toproduce a hermetically sealed compositions of luminescent nanocrystalsare performed in an inert atmosphere, i.e., either in a vacuum and/orwith only N₂ or other inert gas(es) present.

As discussed herein, in suitable embodiments the hermetically sealedluminescent nanocrystal compositions of the present invention are usedin combination with an LED or other light source. Applications for thesesealed nanocrystal/LEDs are well known to those of ordinary skill in theart, and include the following. For example, such sealednanocrystal/LEDs can be used in microprojectors (see, e.g., U.S. Pat.Nos. 7,180,566 and 6,755,563, the disclosures of which are incorporatedby reference herein in their entireties); in applications such ascellular telephones; personal digital assistants (PDAs); personal mediaplayers; gaming devices; laptops; digital versatile disk (DVD) playersand other video output devices; personal color eyewear; and head-up orhead-down (and other) displays for automobiles and airplanes. Inadditional embodiments, the hermetically sealed nanocrystals can be usedin applications such as digital light processor (DLP) projectors.

In additional embodiments, the hermetically sealed compositionsdisclosed throughout can be used to minimize the property of an opticalsystem known as etendue (or how spread out the light is in area andangle). By disposing, layering or otherwise covering (even partiallycovering) an LED or other light source with a composition or containerof the presently claimed invention, and controlling the ratio of theoverall area (e.g., the thickness) of the luminescent nanocrystalcomposition or container to the area (e.g., the thickness) of the LED,the amount or extent of etendue can be minimized, thereby increasing theamount of light captured and emitted. Suitably, the thickness of theluminescent nanocrystal composition or container is less than about ⅕the thickness of the LED layer. For example, the luminescent nanocrystalcomposition or container is less than about ⅙, less than about 1/7, lessthan about ⅛, less than about 1/9, less than about 1/10, less than about1/15 or less than about 1/20 of the thickness of the LED layer.

In still further embodiments, the present invention providesmicrospheres 500, as shown in FIG. 5. Suitably, the microspheres of thepresent invention comprise a central region 502 and a first layer 504 onan outer surface 506 of central region 502, first layer 504 comprisingone or more luminescent nanocrystals 508. The microspheres 500 furthercomprise a barrier layer 512 on an outer surface 510 of first layer 504.

Exemplary microspheres comprising a central region, a first layer, andnanoparticles, as well as methods of producing such microspheres, aredisclosed in U.S. Pat. No. 7,229,690, the disclosure of which isincorporated by reference herein in its entirety.

As disclosed in U.S. Pat. No. 7,229,690, suitably central region 502comprises silica, and first layer 504 comprises an inorganic material,such as silica or titania. Luminescent nanocrystals 508 for inclusion inthe microspheres are disclosed herein, and suitably comprise core-shellluminescent nanocrystals, such as CdSe/ZnS, CdSe/CdS or InP/ZnSnanocrystals. In exemplary embodiments, the luminescent nanocrystals arebetween about 1-10 nm in size.

As described in detail herein, the addition of a barrier layer to thesurface of a composition comprising luminescent nanocrystals provides ahermetic seal on the composition, thus reducing or eliminating thepassage of moisture and/or air to the nanocrystals. Suitably, barrierlayer 512 on microspheres 500 comprises an inorganic layer SiO₂, TiO₂ orAlO₂, though other layers as described herein and known in the art canalso be utilized.

In exemplary embodiments, the microspheres 500 of the present inventionhave a diameter of less than about 500 microns, for example, less thanabout 400 microns, less than about 250 microns, less than about 100microns, less than about 50 microns, less than about 10 microns, or lessthan about 1 micron, including values between these ranges.

The present invention also provides methods of forming microspheres, asshown in flowchart 600 of FIG. 6, with reference to FIG. 5. In step 602of flowchart 600, a particle 502 comprising a first inorganic materialis provided. The particle is then contacted with a compositioncomprising a precursor to a second inorganic material and one or moreluminescent nanocrystals 508, in step 604. In step 606, a peripheralregion 504 is formed on an outer surface 506 of the particle 502, theperipheral region comprising the second inorganic material and theluminescent nanocrystals 508. Then, in step 608, a barrier layer 512 isdisposed on an outer surface 510 of the peripheral region 504.

As noted herein, suitably a silica particle is provided, and theparticle is contacted with an organic material comprising silica ortitania which comprises the luminescent nanocrystals. As describedherein, the luminescent nanocrystals are suitably core-shell luminescentnanocrystals, such as CdSe/ZnS, CdSe/CdS or InP/ZnS nanocrystals with asize of about 1-10 nm. Methods for preparing silica particles andperipheral regions 504 are described throughout U.S. Pat. No. 7,229,690.

Suitably, a barrier layer comprising an inorganic layer, such as SiO₂,TiO₂ or AlO₂ is disposed on the microspheres. As described herein, thebarrier layers can be disposed in various ways, including atomic layerdeposition and sputtering.

Exemplary embodiments of the present invention have been presented. Theinvention is not limited to these examples. These examples are presentedherein for purposes of illustration, and not limitation. Alternatives(including equivalents, extensions, variations, deviations, etc., ofthose described herein) will be apparent to persons skilled in therelevant art(s) based on the teachings contained herein. Suchalternatives fall within the scope and spirit of the invention.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent or patent application wasspecifically and individually indicated to be incorporated by reference.

What is claimed is:
 1. A microsphere comprising: a compositioncomprising: luminescent core-shell nanocrystals; and an inorganic layercovering each of the luminescent core-shell nanocrystals; and a barrierlayer, disposed on the inorganic layer, configured to hermetically sealthe composition, wherein the barrier layer consists essentially of SiO₂,TiO₂, or Al₂O₃.
 2. The microsphere of claim 1, wherein the luminescentcore-shell nanocrystals are separated from each other by the inorganiclayer.
 3. The microsphere of claim 1, wherein the luminescent core-shellnanocrystals are selected from the group consisting of CdSe/ZnS,CdSe/CdS, and InP/ZnS.
 4. The microsphere of claim 1, wherein theluminescent core-shell nanocrystals are between about 1-10 nm in size.5. The microsphere of claim 1, wherein the inorganic layer comprisessilicon.
 6. The microsphere of claim 1, wherein the inorganic layercomprises silica or titania.